fix(fetchers): enforce RSS body size and timeout limits

[chaliy]

Motivation

• Prevent unbounded memory/time use when fetching feeds by applying the same bounded read helpers used by the default fetcher to RSS/Atom responses.

Description

• Import and use read_body_with_timeout, BODY_TIMEOUT, and DEFAULT_MAX_BODY_SIZE from the default fetcher helpers.
• Replace response.text() with read_body_with_timeout(response, BODY_TIMEOUT, max_body_size) and convert the returned bytes with String::from_utf8_lossy so feed parsing respects FetchOptions.max_body_size or the default limit.
• Keep existing redirect/DNS handling via send_request_following_redirects unchanged to minimize behavioral changes.

Testing

• Ran formatter with cargo fmt --all which completed successfully.
• Ran targeted unit tests with cargo test -p fetchkit rss_feed -- --nocapture and all RSS-related tests passed (9 passed; 0 failed).

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Codex Task

[chaliy]

Reviewed. Applies the bounded-read pattern (read_body_with_timeout + DEFAULT_MAX_BODY_SIZE + BODY_TIMEOUT) the default fetcher uses, honoring FetchOptions.max_body_size. CI green across all 11 checks; RSS + SSRF tests pass locally.

Minor follow-up (non-blocking): the _truncated flag is discarded. The default fetcher propagates it into FetchResponse.truncated; doing the same for RSS would keep the API consistent. Out of scope here.

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